The present invention relates to control clusters for a nuclear reactor, each cluster comprising a spider made of a hub fitted with drive shaft fastening means and with fins radiating from a bottom portion of the hub and provided with parallel vertical fingers distributed in a regular array, together with rods provided with plugs fixed removably to the fingers.
At present, control clusters of this type include sixteen fins. Each fin carries a finger at its end. Every other fin also has an intermediate finger. Thus, the cluster includes twenty-four absorbent rods.
Spiders are presently constituted as welded assemblies. The fins are put into place around the hub and brazed. The brazing operation is lengthy and difficult when seeking to avoid any positioning error of the rods and to avoid any deformation due to thermal effects.
In most cases, the fingers are fixed to the fins by respective tenon-and-mortise connections: the end portion of each fin constitutes a tenon which is engaged in a slot of the associated finger. That connection is finished off by brazing. When this solution is adopted, it is impossible to provide a vertical hole passing through the finger, and consequently the plugs of the rods must be fixed in blind holes in the portions of the fingers located beneath the level of the fins. That solution has turned out to be not very satisfactory. French Patent No. 2,599,884 describes a solution that makes it possible to make the rods removable. It is complex and gives the rod assembly a secured connection with the spider. Such a connection is liable to impede sliding of the rods in the guide tubes of an assembly that is to receive the cluster.